Rapid removal of plastic articles from an injection mold can provide benefits such as faster cycle times and article orientation after the demolding process. Part removal systems suitable for use with high speed molds are described in U.S. Pat. No. 4,976,603, European Patent Application 0 415 153 and in PCT Patent Application 93/11923. Each of these disclosures show a pivoting workpiece removal device which incorporates arms which swing in from an outboard position and secure the molded part on suction cups or other vacuum means, at the extremity of the arm. The arms then swing to the outboard position and the mold is ready to close for the next injection cycle. The motive means for the swing of the arms is shown to be a cam and a cam follower plus a linkage assembly or a rack and pinion which directly position the pick-up arms relative to the movement of the opening mold. This approach carries with it a variety of disadvantages.
Firstly, engineering the proposed designs can be a laborious task of generating an optimized cam profile with accompanying linkage or gearing components which serve to move the arms into an open mold, pick up the article and move the arms to the outboard position. If the cam profile is aggressive, causing high acceleration and deceleration rates, the inertia of the moving masses will soon result in damaged or prematurely worn components. If the profile is to be made less aggressive, thereby reducing acceleration and deceleration rates, extra time will be spent for the mold to travel a further linear distance in its opening stroke than is actually required for clearing the swing arm motion. This results in a disadvantageously longer cycle time. Furthermore, the injection molding machine platens may not permit sufficiently long cams to be incorporated due to interference between the cams and the platens. Increased machine opening stroke requirement is also of concern, especially if a multilevel stack mold with more than one molding face is to be installed.
Reducing the moving masses of cams and cam followers to decrease the effect of inertia has to be balanced against required structural strength and rigidity.
A further disadvantage in the cam and cam follower system is that such a system does not take into account that the linear stroke of the mold halves must be larger when the retrieval arm is loaded and that less clearance is required when the empty retrieval device is in route to a product pick up position.
That is, in the present invention the opening stroke of the mold halves must be of sufficient linear distance to provide clearance for a loaded retriever to swing outwardly freely and without interference to a discharge position.
Obviously, an empty retriever requires less clearance. Therefore, the timing of the motion of the retriever in the empty condition can be set so that in the empty condition the retrieval can be programmed to swing inwardly to a loading position before the mold opening stroke is completed. Therefore, the outward swing of a loaded retriever requires a mold linear opening stroke which is a function of the size of the molded article while the inward swing of an empty retriever requires less linear space and occurs when the mold opening stroke reaches a predetermined or fixed linear dimension in the course of the mold opening stroke.
Consequently, the prior art cam operated retrieval devices waste cycle time by activating the retrieval device, loaded or unloaded, at the same linear mold opening strokes.
Additional cycle time is lost because of the nature of a cam system. For example, in a cam system, which is responsive to the motion and position of the mold halves, the mold opening stroke is not governed or limited merely by the clearance required for an empty retrieval arm to enter the open mold; the system requires continued motion of the mold halves in the opening direction effective to continue cam action to drive the retrieval arm into the open mold. Obviously, this occurrence is a waste of cycle time.
Adjustability of prior art systems also presents a difficulty. Since all the elements of the system integrate mechanically, a change in movement speed or position requires replacement components appropriately designed and precisely manufactured. If even one of the various components of the system is incorrectly sized or positioned, the adverse affect could cause improper or ineffective article removal. This situation can cause numerous delays and expense in the "fine tuning" of the operating system.
In addition, there is a lack of flexibility in a mechanical approach. As each mold and molded article can have a different shape or size, the cam and linkage assembly must be individually engineered for optimal function. Convenient and economical interchangeability of componentry between similar but not identical molds is thereby restricted.
A further disadvantage of the prior art is found in the space that it occupies at the top and bottom of the mold. Cams, linkages and gears all occupy an area which otherwise would be used for mold services such as air, water and electrical supply. This adds complication and engineering effort to reroute these services to less desirable connection points. In some cases, it may be mandatory for the services to be located on the mold such that the cam and linkage assembly must be spaced from the mold using extensions and adapters. This can disadvantageously add significant height to the overall mold assembly causing further mold handling difficulties.